The present invention relates generally to a location system for locating mobile objects such as people on foot, cyclists, vehicles and the like in an emergency.
In U.S. Pat. Nos. 4,891,650 and 5,055,851 of Sheffer a vehicle locating system was described which relied on the use of the existing nationwide cellular radio network used in mobile telecommunications (cellular phone system). Cellular telephone networks rely on a fixed array of cell sites, each cell site covering a predetermined area and having a wireless signal detecting and generating unit at a central location in the area. These cell sites receive and transmit signals to and from cellular phones within their area on their own set of frequencies, and are linked via conventional land lines to a mobile telecommunication switching office (MTSO) for transmission to other cell sites, receivers, or conventional phones via a telephone company central office (CO).
U.S. Pat. Nos. 4,891,650 and 5,055,851 describe a system for detecting an emergency in a vehicle, such as forced entry, and using the existing cellular radio network to transmit an alarm signal from the vehicle to adjacent cell sites. The mobile telephone switching or MTSO is modified in order to transmit a signal indicating the signal strength of the received alarm signal to an external, to telephone company, central alarm station. The transmitted signal also includes vehicle identifying information. At the central alarm station, the approximate vehicle position can be estimated based on the relative strengths of the signal received from the vehicle at adjacent cell sites. A police car can then be dispatched towards the approximate vehicle location. The tracking vehicle turns on its own cellular car radio so that the tracking vehicle is itself tracked using the same technique. In this way, an operator in the central alarm monitoring station can observe the relative position of the original vehicle and the tracking vehicle or vehicles. Since the exact position of the tracking vehicle is known, this can be used to provide the tracking vehicle with instructions for moving towards the target.
One disadvantage of this previous vehicle location system is that it requires modification of the software at the MTSO so that it will generate the required signal strength signals for transmission to the central monitoring station. This would be a relatively complex and expensive procedure.
In my U.S. Pat. No. 5,218,367, a vehicle tracking system is described in which signal processing units comprising modified cellular transceivers are installed in vehicles and are connected to one or more vehicle break-in sensors. The vehicle unit initiates an emergency transmission to a remote monitoring station in the event of an emergency, and also monitors signal transmissions from adjacent cell sites and transmits cell site identifying information in the emergency transmission.